With reference to FIGS. 1a, 1b and 1c, a junction section (JS) 4 of a conventional semiconductor waveguide Y-junction splitter 1 receives an input beam at an input port 2 for transmission to an input waveguide 3, and splits the input beam into two output beams onto two output waveguides 6 and 7 for output to two output ports 8 and 9, respectively. Typically, the input port 2 and the output ports 8 and 9 are optically coupled to external waveguides (not shown) for transmitting the optical beams to and from the Y-junction splitter 1. A good Y-junction splitter is characterized by low insertion loss (IL), i.e. the amount of power lost through the Y-junction splitter that does not go to the output waveguides 8 and 9; low return loss (RL), the amount of light reflected by the JS 4; and good split ratio, e.g. a balanced Y-junction splits evenly 50:50, not 51:49.
One problem that arises, especially with a high-index contrast platform, such as Si/SiO2 or SiN/SiO2, is that there can be an abrupt change in mode profile between the optical mode guided just before the JS 4 and the optical mode just after the JS 4. The abrupt change results in exciting multiple modes past the JS 4, such as high order guided modes or radiation modes. These parasitic modes can lead to high IL or RL.
Another problem arises with the design of unbalanced Y-junctions splitters. Although balanced Y-junction splitters are intuitively designed by symmetry, designing an unbalanced Y-junction splitter with an arbitrary split ratio is non-trivial; especially when low IL is required.
An object of the present invention is to overcome the shortcomings of the prior art by providing a more efficient Y-junction waveguide splitter.